Bark Espionage.

This is the Tree Listening Installation. It is a small electronic listening device built for eavesdropping on the inner acoustics of trees. How does it work? Glad you asked, the device is placed on the trunk of a given tree and then connected to as many as ten sets of headphones which hang down from the tree's canopy. Botany becomes your iPod.
"This allows the public to listen 'live' to the sound of water being pulled up from the roots to the leaves through the xylem tube," Alex Metcalf, the device creator, writes.
As he explained in an interview with the Guardian last week: "The technology for this is usually invasive. You bore into the tree and take away a section, then seal in a listening device. The thing about my device is that you don't have to cause any damage, and you can listen to any tree, anywhere, any time – plus you can do it long term. Cutting a hole in a tree means you are wounding and infecting it, which will affect the recording." I've never heard the sound of the inner acoustics of a tree, nor have I heard the sound of water being pulled up from the leaves through a tree's xylem tube.. but it sounds incredibly relaxing.